The present invention relates to a new and improved construction of a solar collector composed of a transparent, evacuated cover tube or pipe having arranged therein absorbers and a reflection layer at least partially covering the inner surface of the cover tube.
In order to convert the solar energy into thermal energy, there are employed collectors which directly infeed the energy to a heat storage or the load or consumer. The power capability of the total solar collector installation is predominantly determined by the effectiveness of the solar collectors. During the design of solar collectors the most important criterion is to have as high as possible absorption factor of the absorber and as high as possible transmission factor of the transparent covering. Furthermore, the inherent losses of the collector must be maintained as small as possible and in practice should not exceed values between 50 to 100 W/m.sup.2 (watts per square meter).
With the heretofore known solar energy systems there are presently employed predominantly two different basic types of collectors. In particular, firstly the so-called flat plate collectors wherein there is used a glass plate and an energy-absorbing surface arranged therebelow and in spaced relationship from the glass plate; in other words, the energy-absorbing surface is remote from the side of the sun. At the rear face of the absorbing surface, there circulates the heat exchange medium in a gaseous or liquid state. A second known embodiment consists of a double-wall glass tube or pipe collector in which an outer glass tube is connected to and sealed-off in relation to an inner glass tube arranged at a spacing therefrom. The space between both of the glass tubes is evacuated. At least the inner tube has an energy-absorbing layer, and specifically over the surface which is exposed to the action of the sun and in the absorber there is located a heat-absorbing liquid which receives the thermal energy of the sun. A number of such type of tubular-shaped collectors are connected to a collecting line or conduit system through which the liquid is delivered to the tubes and through which, on the other hand, the heat, given off by the absorber, of the liquid contained therein, is delivered to a heat exchanger or load or into a heat storage.
Furthermore, there have been used for quite some time solar collectors having absorbers wherein such absorbers are equipped with a heat-resistant, blackened layer. The inner sides of a hollow cylinder are formed to be light refractive at the side confronting the incidence of the solar energy, so that the major part of the incident solar radiation impinges in a focused manner at the side surfaces of the absorber tube confronting the sun.
Some of the more notable drawbacks of the heretofore known arrangements particularly are in terms of the relatively large constructional expenditure which is required, and, furthermore, the high radiation losses which arise due to the arrangement of a number of transparent tubes. These radiation losses result, for instance, by virtue of the fact that, in the case of glass with perpendicular incidence of the solar radiation approximately four percent of the boundary surface is reflected, i.e., eight percent for each individual pane. With light incidence below 30.degree. in relation to the glass pane, the reflection rapidly assumes higher values. Such type reflection losses can basically be reduced by providing a reflection-reducing coating, but such measures are expensive and complicated.
Apart from the radiation losses there are also to be considered the losses due to heat conduction of the air as the next greatest loss coefficient. For this reason for quite some time it has been proposed to evacuate the intermediate space between the outer cover tube and the absorber.
Yet, there are always present with this proposal problems as concerns the seal between the cover tube and the absorber. By virtue of the different diameters between the cover tube and the diameter of the absorber, there are difficulties which arise owing to the different coefficients of expansion of both tubes.
Hence, with the foregoing in mind it is a primary object of the present invention to provide a new and improved construction of a solar collector which does not exhibit the aforementioned drawbacks, which is simple and inexpensive to fabricate, and which can operate over long periods of time essentially without any need for maintenance.
Still a further significant object of the present invention aims at the provision of a new and improved construction of solar collector which is relatively simple in construction and design, economical to manufacture, and operates at improved efficiencies.
Now in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the solar collector of the present development is manifested by the features that the absorber extends over a part of the inner diameter of the cover tube and the absorber is provided with liquid-conducting lines as well as a holder plate carrying such lines and which is provided with a selective layer. Further, the individual cover tubes are arranged at a likewise evacuated collector line or conduit.
The advantage of the inventive construction especially resides in the absorber together with the holder or support plate carrying the same. At which holder plate there is applied a selective layer, which can be arranged in an evacuated cover tube or pipe, and the holder plate with the selective layer is completely enclosed by the absorber tube, so that none of the the heat radiation is lost, and thus, there can be obtained low thermal losses and a high thermal capacity.
According to a further advantageous construction of the subject matter of the invention, there are arranged between the individual cover tubes which receive the absorbers double-wall line connections composed of connection tubes and liquid lines or conduits and the intermediate spaces between the liquid lines or conduits and the outer connection tubes are likewise evacuated.
In this way there is realized the advantage that the individual collector tubes can be mutally interconnected in a most simple manner, and both the intermediate spaces between the cover tubes and the actual absorber and the connection tube likewise are evacuated, so that the smallest heat losses prevail in the line connections.
Furthermore, it is advantageous if the evacuated intermediate spaces of the cover tubes are connected with one another by means of the connection tubes.
In this way there is realized the further advantage that there can be undertaken at a single location of an entire solar collector installation possible post-evacuation operations, without each individual collector tube having to be itself evacuated.
According to a further possible construction of the invention the reflection in the cover tube below the absorber possesses a rib-shaped raised portion or protuberance extending over the entire length of the cover tube.
Due to the arrangement of a reflection layer in the cover tube, wherein this layer has a rib-shaped raised portion or arrangement, which is preferably located below the absorber and extends over the entire length of the cover tube, there is also directed the indirect or diffuse radiation of the sun at the underside of the liquid-carrying lines or conduits, so that also with diffuse radiation there is accomplished an appropriate utilization of the energy.